I am very optomistic about the probability of life occuring on other, extra-solar planets. But of the search for 'intelligent' extra terrestrial life, I am skeptical.
For those of you who aren't familiar with the Drake equation, it is an equation which attempts to quantify how many planets are home to intelligent life in our galaxy (or our universe, or however far you dare to estimate). The equation is a simple multiplication of unknowns. It looks like this:
N = R* X fp X ne X fℓ X fi X fc X L
N is the number of civilizations in our galaxy with which communication might be possible;
R* is the average rate of star formation in our galaxy
fp is the fraction of those stars that have planets
ne is the average number of planets that can potentially support life per star that has planets
fℓ is the fraction of the above that actually go on to develop life at some point
fi is the fraction of the above that actually go on to develop intelligent life
fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L is the length of time such civilizations release detectable signals into space.
When we look at the Drake equation, there is a glaring component that, depending upon interpretation, can decidedly make or break the probability and hope of finding intelligent life: What percentage of planets that develop life will develop intelligent life?
How do we define intelligence? Are cells with organelles surrounded by a protective cell walls intelligent? Multicellular creatures? Is intelligence the point at which decidedly living creatures interact with decidedly non-living things apart from themselves in ways that demonstrate foresight and imagination? Or when organisms talk on cell-phones?
First of all, we have to realize that some things we generally think of as ubiquitous to life are actually highly complex and specialized for our unique and singular time and place in the universe. For instance, something like a run-of-the-mill walled cell seems to be pretty far down the line of ancestral lineage, but in the scheme of even Earth's geological time-scale, The first cell with DNA and a cell wall could be thought of as a relatively recent ancestor. A cell is an advanced form of life that had already taken many steps to reach its form, and could have taken many different routes than it did, resulting in a drastically different 'building block of life'. DNA itself is a highly developed, complex replicating system that requires complex biochemical machinery to operate. We can imagine drastically different replicating systems.
Richard Dawkins pinpoints the origin of life as the origin of true heredity. He explains, "The key to true heredity is that each replicator resembles the one from which it was copied more that it resembles a random member of the population." It is under these circumstances that natural selection can direct populations to change relative to each other and specialize into distinct niches. We can imagine heredity in many forms, and there are even more which we cannot imagine. And, taking these drastically different points to start branching off of, it seems that life could develop in an infinite amount of ways for a long time, becoming very advanced, without ever creating anything like a walled cell, for instance.
With this in mind, we can return to the question: How likely is it that planets that develop life will develop intelligent life? First, we must define intelligence. This, of course, is generally considered to be an extremely difficult question to answer, but there is a very straightforward definition that has already been quietly agreed to for mostly practical reasons: Intelligent life forms are ones that send out signals in the electromagnetic spectrum that are irregular and strong enough to make human observers pay attention. (One caveat to that definition would the inclusion of physical messages or personal appearances.) These are the only forms of life that we currently have the ability to look for, but more importantly, it is technology that is considered universally by humans to be the benchmark of intelligence.
Now, to rephrase the question: How likely is the behavior of developing devices that communicate in the electromagnetic spectrum? Let's think about it. This would require the behavior of developing technologies in general. And this in turn would require the development of a large brain, which would have to evolve from nerve ganglia, and on and on. Keeping in mind the almost infinite imaginable forms of heredity, whose initial spark could send natural selection off in so many inconceivable and unimaginable ways, a technological society seem highly unlikely.
One might argue that there are multiple paths to technological development other than concentrated nerve centers like the brain. I would reply that technology is an extended phenotype that is unique to the brain and its interactions with limbs, and that independently evolving life, developing in the presence of drastically different environments could coincidentally develop something very closely resembling the brain, but the possibility is negligible.
My point is that most people, when discussing ET life, and things like the Drake equation, take technology for granted as the eventual product of natural selection, with an argument like this in mind, "Whatever the path, given enough time, groups of evolving individuals will start to create technologies, because the intentional manipulation of one's environment provides such a distinct advantage that the niche must eventually become occupied in virtually any environment."
The argument is a sound one, and it is the stance I would normally take. But perhaps this view is a false one driven, forgivably, by our biased stance as a species who have 'achieved' such behavior. Perhaps technology is merely another obscure behavior development, like salmon swimming upstream, or walking bipedally, or building nests from scraps and hatching eggs, that arose to fulfill a very specific niche provided by a very specific environment shaped by many previous interacting niches. Just as I don't think transforming flower pollen into honey is an inevitable niche that will be filled by highly evolved life, I don't think the development of large brains, or the creation of technology is an eventual inevitability. Organisms change their physical structure and behavior because of need. The only purpose is the propagation of genes. I would imagine that even on planets that do develop complex life, genes could propagate until their closes stars destroy them without ever needing to create tools to survive. Even on our own planet, with the exact same replay of all of the infinitely unlikely combinations of events that led up to Earth 65 million years ago, we could change one chance even like preventing the meteor that exterminated the dinosaurs could possibly have changed the future so that developing large brains was never necessary, or allowed given the circumstances. In a hypothetical Earth, with the exact same location relative to other celestial bodies, exactly the same history of formation with the same distribution of elements, tidal forces, arrangement of land masses etc., with the exact same history of life except one rock avoiding collision with Earth. Even in this hypothetical situation, we can imagine not having a technological species.
Of course, although astronomers disagree on exactly how big the universe is, one thing is for certain: the universe is super big, and there's a lot going on inside of it. The point of this essay is not to argue that there is no possibility of intelligent life on other planets, even in the most specific sense of the word 'intelligent'. The point is that, perhaps even when we think we are considering the nature of life in an objective way, we are still holding a bias that represents an extremely low rate of probability. After all, even on our planet, we are the only species among millions to display the type of behavior that we are looking for in the cosmos.